x2a2+y2b2=1\dfrac{x^2}{a^2} + \dfrac{y^2}{b^2} = 1a2x2+b2y2=1: foci (±c,0)(\pm c, 0)(±c,0) where c2=a2−b2c^2 = a^2 - b^2c2=a2−b2; eccentricity e=c/a<1e = c/a < 1e=c/a<1.